Recently, various techniques for analyzing biometric data of the body using a method for measuring the optical properties of a turbid medium are being developed. These techniques have attracted a lot of attention in that they are non-invasive and can provide biometric data, and a lot of attention is being focused on research for development into entry-level devices according to the needs of consumers.
These techniques generally calculate the concentration of a chromophore in the turbid medium by measuring the absorption coefficient and the scattering coefficient of a turbid medium in a near infrared ray region. There are three methods known to measure the absorption coefficient and the scattering coefficient of a turbid medium. Specifically, these methods include a steady-state (SS) method of irradiating light of a predetermined intensity into a turbid medium and calculating the concentration of a chromophore according to a multi-distance measurement method, a frequency domain (FD) method of measuring a changed amplitude and phase for a modulated light source, and a time domain (TD) method of measuring a change over time for a pulse-type light source.
The SS method does not require the modulation of light or pulse generation and thus does not require a detector that decomposes light reflected from a turbid medium by frequency domain or time domain. Therefore, the SS method is cheaper than the other methods (i.e., FD method or TD method). However, the SS method uses the multi-distance measurement method to separate the absorption coefficient and the scattering coefficient. Therefore, in biological tissue with high non-uniformity, the SS method is more likely to generate distortion during analysis than the other methods.
The TD method and the FD method do not use multi-distance measurement method and thus are more suitable for biological tissue with non-uniformity than the SS method. However, the TD method and the FD method require a detector configured to detect pulse generation or frequency-modulated light source and the properties thereof. Therefore, the TD method and the FD method have shortcomings in terms of implementation and cost.
The present disclosure adopts the steady state (SS) method but uses a lock-in amplifier structure to minimize the effect of ambient light and implement a high signal-to-noise ratio (SNR). The lock-in amplifier refers to an amplifier configured to recover a signal in noise and has been used to remove a noise which is much greater than a signal to be detected. The lock-in amplifier may multiply a target signal with a specific frequency and a reference signal with the same frequency as the target signal, to extract a magnitude of the target signal. For example, if a noise is included in a broad frequency band including a frequency (fa) of the signal to be detected, the signal to be detected and the reference signal having the same frequency as the signal to be detected are multiplied to obtain a harmonic wave (2fa) which is the sum of the two frequencies and a direct current (DC) which is the difference between the frequencies. An intensity of the direct current (DC) which is the difference between the frequencies is proportional to an amplitude of the signal to be detected. If a low-pass filter is applied to the signal obtained in this way, the sum of the frequencies is removed and only the difference between the frequencies is obtained. As such, if a signal only in a direct current (DC) band is detected using the lock-in amplifier, a level of the signal to be detected is not changed but a magnitude of the noise is decreased, and, thus, it is possible to effectively remove a noise generated outside the detection device.
However, in order to effectively remove a noise by a bio-signal analyzing apparatus including the above-described multiple light sources and multiple light detectors, a new method is needed.